Seismic data is used to visualize the ground. This is achieved by measuring travel times of reflections from pressure waves propagating through the ground. Seismic data and interpretations of seismic data will therefore often have z values in travel time, not depth. In order to get a depth value for the travel time values, we need to do a depth conversion. A depth conversion combines the velocity of the pressure wave with the travel time to get a depth value.
Data sets may be depth converted in Geocap using the command "Velocity Model Depth Convert". This command uses a "Velocity Model" as input in order to determine the velocity of a the pressure wave.
Velocity Model
A velocity model is stored as an item your Project. I represents the seismic velocities in an area, and is used as input to the "Velocity Model Depth Convert" command. In order to create a velocity model, set a "Velocity models" schema on a generic folder, and run the command "Generate Velocity model". In order to see or edit the parameters of a velocity model, right click the velocity model item in the project and select "View/Edit".
Geocap currently supports these types of velocity models:
Constant Ground Velocity
The constant ground velocity model is a single layer velocity model. It uses one constant ground velocity. Input to the velocity model is water velocity, the seabed in time, and the ground velocity. The Seabed data may be interpreted lines from 2D seismic (IHorizon or PolyData) or grids (StructuredPoints).
Layer Velocity model
In the Layer Velocity model, you may use seismic horizons as interval velocity boundaries. The horizons may be interpreted lines from 2D seismic (IHorizon or PolyData) or grids (StructuredPoints). You may set the interval velocity of each interval using a table. In the table you also need to set the top horizon of each interval. In addition you need to set the water velocity as input.
When going from top to bottom, the layer velocity model will initially use the water velocity as interval velocity until it meets the top horizon. From the top horizon, it will use the top horizon velocity until it meets the next horizon, and so on.
RMS velocity (Stacing velocities)
The RMS velocitiy model uses RMS velocity data sets as input. It is possible to select a single data set in the project as input, but you may also select a folder containing multiple RMS velocity data sets as input. In some cases Stacking velocities may be used as RMS velocities.
When the velocity model computes the velocity of a point, it will use the RMS velocity points within a search radius as input. The RMS velocity data set, the interpolation method, and the search radius are input to the RMS velocity model.
In order to generate an average velocity, the velocity model will run Dix formula on the RMS velocity to generate interval velocities. An average velocity is computed from the interval velocities.
Note that the Search radius should be big enough so that all points where you need a velocity will have RMS points within the search radius. However if the search radius is set too big, the velocity computation for a point will include too many RMS points, and the calculation will be slow. The radius will vary depending on the density of the RMS velocity dataset.
Average velocity cube
An average velocity cube is a 3D raster which holds the average velocity at the cube nodes. The cube may be generated in Geocap, or it may be imported from other software. Such a cube is used as input for the Average Velocity cube velocity model. You may also specify a seabed surface and a water velocity which will be used above the seabed surface. The cube will be used to look up velocities below the seabed surface.
Specifying a seabed surface is optional. If a seabed is not specified, the velocity model will only look up velocities in the cube.
The velocity model will clamp the bottom velocity in the velocity cube as velocity below the cube.
Depth Conversion
The command "Velocity Model Depth Convert" can be used to depth convert data in Geocap. The command is available on a velocity model. Right click the velocity model you want to use, and select "Velocity Model Depth Convert".
The command can be used to depth convert many different datatypes in Geocap, including:
- Seismic 2D lines
- IHorizon (seismic interpretations)
- Poly Data (points, lines, polygons)
- Structured Points (Grids)
In the input field it is possible to browse in multiple data sets. You may also specify folders as input. If this is case, all the data sets in the folder will be depth converted. When the input field has folders, the same folders will be created in the output folder.
In order for the command to succeed, make sure that the output folder can contain the input data sets. Please note that a "Generic" folder cannot hold
seismic data set directly. However a "Generic" folder can hold a "Seismic Data" folder, which again can hold seismic data sets.
The depth conversion command has options to store the Average velocity used to do the depth conversion of each point in the result. You may also choose to store the "instantaneous Velocity". The "Instantaneous Velocity" is computed by looking at the change of average velocity from the velocity model around this point. For seismic lines the the "Instantaneous Velocity" is the interval velocity of a single sample in the seismic. Storing and visualizing these velocities will make you able to quality control the velocity model used for the depth conversion.
If you store velocities when depth converting seismic lines, the velocity values are stored as seismic attributes to the resulting seismic lines. You may visualize the velocities with the regular "Seismic Display" command. Note that storing velocities will make the depth conversion of seismic lines slower.